As known, an epicyclic transmission comprises a sun gear, a ring gear and a plurality of planet gears, which are interposed between the sun gear and the ring gear and are supported by a planet carrier. A transmission of this type can transmit motion between coaxial shafts, rotating at different speeds, and is very effective in implementing such a function while keeping weight and cost low. Epicyclic transmissions are widely used also in aeronautic engines, e.g. to transmit motion to the fan in turbofan engines.
In most applications, the planet carrier is of the asymmetric type, i.e. comprises two substantially plate-like elements arranged on parts axially opposite to the planet gears and fixed to each other by means of a plurality of crossbars or tenons. One of the two plate-like elements is then integrally connected to a support structure, if the planet carrier is fixed.
Each planet gear is coupled to the planet carrier by means of a respective support pin, the opposite ends of which are inserted and locked in the plate-like elements. In particular, the planet gear is coupled to an intermediate portion of such a support pin by means of either a sliding bearing or a rolling bearing, e.g. of the roller type.
During the operation of the transmission, the forces transferred by the planet gears to the respective support pins in general deform the planet carrier, and consequently tend to cause a displacement of the two plate-like elements with respect to each other. The tenons and the supporting pins are both deformed as a result of such a displacement. In particular, the support pin axes pass from an ideal operating condition, coinciding with a resting condition, in which the axes themselves are parallel to the axis of the sun gear and of the ring gear, to a real operating condition, in which they are inclined by an angle other than zero, variable as a function of the entity of the transmitted forces, and thus of the deformation of the planet carrier.
This swiveling of the support pin axes, and thus of the rotation axes of the respective planet gears, with respect to a condition of parallelism with the axis of the ring gear and of the sun gear produces a lack of uniformity in the distribution of contact pressures in the meshing zones of the teeth of the planet gears with the teeth of the sun gear and of the gear ring. As a consequence, a general malfunction of the transmission and also rapid wear of the components, which are in contact and in relative motion within the transmission itself, occur.
In order to avoid these drawbacks, a ball joint is provided for each planet gear which couples the inner ring of the bearing with the support pin and tends to compensate for the difference of swiveling between the rotation axis of the planet gear and the axis of the support pin itself in some known solutions.
However, this type of solution is not satisfactory because the inner ring of the bearing rotates with respect to the support pin about the axis of the latter, at least in some operating conditions. It is appropriate to lock this relative rotation to prevent loss of function of the bearing, i.e. that of supporting the planet gear as it rolls. Such a function would instead be performed by another component, i.e. the ball joint, which is sized only to allow the self-alignment of the planet gear with the meshing zones.
Technical devices may be adopted to lock the relative rotation of the inner ring of the bearing with respect to the ball joint without losing the self-alignment function performed by the latter, but such technical devices tend to wear very prematurely and are thus not reliable, and above all increase complexity of the transmission.